Help with mask

if I understand you, I think you want to want to track the mask with the glass, or better try to post some screenshots to explain your idea better 

Pre-compose the masking, apply your Shatter effect or whatever in the parent comp.

Mylenium

It will be better if you showed your Comp Panel and Timeline Layers and the mask(s) you've applied. This will allow us insights into the shot, the layers you have, the layers related to the effect you are trying to achieve and the mask(s) you have and how they are used.